The present invention relates to a cathode structure provided in an electron gun of a cathode-ray tube used in a television, a computer monitor, or the like.
As shown in FIG. 2, a cathode-ray tube 1 includes a faceplate portion 3 having a phosphor screen 2 on its inner face, a funnel portion 4 bonded at the rear of the faceplate portion 3, and an electron gun 6 for emitting electron beams 5 placed inside a neck portion 7 of the funnel portion 4.
An indirectly heated cathode structure 108 is provided at an end of an electron gun. As shown in FIG. 8, in the cathode structure 108, one end of a cylindrical sleeve 109 is covered with a cap-like base 110, and an electron-emissive material layer 111 is formed on the surface of the base 110. The electron-emissive material layer 111 is formed of an electron emitter for emitting thermoelectrons. A coiled heater 115 is provided inside the cylindrical sleeve 109 and includes an alumina electrical insulating layer 113 on a metal-wire coil 112 and a dark layer 114 as an upper layer thereof. Generally, the electron-emissive material layer 111 is formed on the whole base surface 120 facing an electron emitting side.
A cathode structure also has been proposed in which an electron-emissive material layer containing alkaline-earth metal or the like is allowed to adhere only to the center portion of a base surface by spraying or the like (JP 5(1993)-334954 A). In this cathode structure, the electron-emissive material layer located at the periphery, which does not participate much in electron emission, is omitted, so that the heat from a heater can be absorbed efficiently by the electron-emissive material layer.
In a step of activating a cathode, a reducing element (for instance, magnesium, silicon, or the like) contained in the base diffuses thermally to the interface between the electron-emissive material and the base, reduces the electron-emissive material (whose main component is an alkaline earth oxide such as barium oxide), and thus produces free barium. This enables electron emission. This reductive reaction is expressed by the following equations:
2 BaO+1/2 Si=Ba+(1/2)Ba2SiO4
and
BaO+Mg=Ba+MgO.
In the conventional cathode structure described above, however, there have been problems in that sufficient electron emission cannot be obtained at an initial activating step and a decrease in electron emission during operation with the passage of time is worsened. In addition, there also has been a problem in that excessive shrinkage of the electron-emissive material layer is caused during operation due to the progress of the reductive reaction and this increases variations in cut-off voltage (electron beam erase voltage) inversely proportional to the distance between a counter electrode and the electron-emissive material.
According to the investigation by the present inventor, from an entirely different viewpoint from the improvement in thermal efficiency as described in JP 5(1993)-334954 A, it was found that the above-mentioned reductive reaction progressed suitably when the amount of the electron-emissive material and the size of the base were adjusted to satisfy predetermined relationships and thus the above-mentioned problems were solved.
The present invention is intended to provide a cathode structure with characteristics improved by optimization of the relationship between the sizes of a base and an electron-emissive material layer.
An embodiment of a cathode structure according to the present invention is a cathode structure for a cathode-ray tube having an electron-emissive material layer formed on a base containing a reducing element. The cathode structure is characterized by satisfying the relationship of 0.24xe2x89xa6B/A less than 0.93, wherein A denotes an area of a surface for layer formation of the base and B represents an area where the base and the electron-emissive material layer are in contact with each other, and having a zero-electric-field saturation current density of at least 6.4 A/cm2 after an implementation of an accelerated life test for 5000 hours under conditions of a vacuum of 10xe2x88x927 mmHg, a cathode temperature of 820xc2x0 C., and a current led out from a cathode of DC 300 xcexcA.
In this context, the surface for layer formation of the base refers to the surface of the base facing the electron emission side and does not include side faces of the base. When the surface for layer formation has a circular shape, the area of this surface can be determined by a formula of Π(d/2)2 based on its diameter d.
According to this cathode structure, a practically sufficient cathode current can be obtained even after long-term use, and in addition, the variations in initial cathode current among cathodes can be reduced considerably. When the size of the base is determined, the size of the electron-emissive material layer required for a practical operation can be determined easily.
Furthermore, another embodiment of a cathode structure of the present invention is a cathode with an electron-emissive material layer formed on a base containing a reducing element. The cathode is characterized by satisfying the relationships of 0.24xe2x89xa6B/Axe2x89xa60.93 and 0.4xe2x89xa6D/Cxe2x89xa60.7, wherein A denotes an area of a surface for layer formation of the base, B an area where the base and the electron-emissive material layer are in contact with each other, C the thickness of the base, and D the thickness of the electron-emissive material layer. This cathode structure has a long life and allows variations in cut-off voltage to be reduced.